a-short-review-of-histology

Motor Units:

A motor unit is a (motor) nerve plus the muscle cells that the nerve innervates
Motor units may be small, in which case few muscle cells are innervated by the one nerve
Motor units may be large, in which case many muscle cells are innervated by the nerve
When the nerve is stimulated, all the muscle cells in the motor unit will contract
Functionally, small motor units allow for greater control and are typically found in regions where delicate or fine movements are required eg muscles which move the eye, the small muscles of the hand which are involved in fine hand movements
Large motor units have less control and are typically found in large muscles, eg gluteus maximus, deltoid, where powerful movements are performed
Individual muscles have many motor units and this allows the muscles to provide a range of movement and/or a range of power
It is this which allows us to use the same muscle to be delicate when holding an egg or provide strong grip when lifting a heavy weight
Motor units also allow us to use different parts of a muscle
Deltoid, is an abductor of the shoulder joint when all, or an even selection of motor units is used

Cardiac Muscle Overview

Small motor unit - one axon supplies few muscle fibres

Large motor unit - one axon supplies many muscle fibres